The Severn River is a river in northern Ontario. The northern Ontario river has its headwaters near the western border of the province. From the head of the Black Birch River, the Severn River is long and its drainage basin is 102,800 km² (39,700 mile²), a small portion of which is in Manitoba. Its source is Deer Lake and flows northeasterly into Severn Lake, then by a second section to Hudson Bay where it ends at Fort Severn.